In a conventional gas fired water heater, a gas burner is located beneath the lower end of the water tank in a header space. A central vertical flue is provided through the tank for discharge of the waste combustion or flue gases. Water in the tank is heated by heat from the burner in the lower header space and from the waste gases passing upwardly through the central flue.
Considerable heat losses occur during non-use or standby periods with conventional gas fired water heaters. Since all of the water in the tank is to be heated to the same pre-set temperature, large amounts of energy are expended to keep this large volume of water at this high temperature. Also, heated water rises to the top of the tank by convection causing the water to stratify and become overheated at the top of the tank. Drawing limited amounts of water from the top of the tank may result in overheated water being delivered. With a single central flue, although some increase in heating efficiency is gained by heat transfer from the flue gases, this is limited to that amount of heat which can be transferred to the water in close proximity to the central flue.
U.S. Pat. No. 4,676,199, issued June 30, 1987 to Daugirda et al. and assigned to Rheem Manufacturing Company discloses a multistage, high efficiency, gas fired water heater including a lower primary heating tank and an upper secondary heating tank. A plurality of primary flues are provided through the primary tank which are aligned with secondary flues in the secondary tank. Manifolds are situated above, below and between the tanks. One of the primary flues is constructed to operate as a condensate removal pipe for draining water from the secondary flues. This water heater design is complicated by the provision of both primary and secondary flues in both tanks. The flues must be aligned and of different diameters for proper functioning.